1. Field of the Invention
This invention relates to a system for cleaning the exhaust gas generated from the internal combustion engine, and more particularly to the exhaust gas cleaning system capable of removing the oxygen molecules in the exhaust gas to convert it into the reducing gas and cleaning thereafter nitrogen oxides.
2. Description of the Prior Art
It is well known that injurious carbon monoxide, hydrocarbons and nitrogen oxides (NOx) are contained in the exhaust gas of the automobile engine or the like internal combustion engine and that the former two, i.e., the carbon monoxide and the hydrocarbons, can be removed by the thermal reactor or the catalyzer cleaner which combustions again causes combustion of the exhaust gas at high temperature. Although the nitrogen oxides can be removed by the catalyzer reactor, the high cleaning efficiency is not expected to discharge most of the nitrogen oxides and to cause the photochemical smog.
Two processes have been proposed for removing the nitrogen oxides from the exhaust gas. In one, the catalyzer decomposes the nitrogen oxides into nitrogen and oxygen. In the other, the nitrogen oxides are reacted with carbon monoxide, hydrocarbons and hydrogen in the exhaust gas. The former process requires a large amount of catalyzer due to the slow reacting rate and accordingly is not useful for treating large volumes of the exhaust gas discharged, for example, from the automobile engine. On the other hand, with the latter process it is substantially impossible to remove the nitrogen oxide, as the oxygen in the exhaust gas suppresses a reaction between the nitrogen oxide and the carbon monoxide.
Cleaning of exhaust gases and reducing the fuel consumption have recently become necessary in view of the pollution problems and the shortage of the fuel. In the case where fuel-air ratio of the gas mixture to be introduced into the engine is lean, e.g., a ratio higher than 15, to reduce the fuel consumption, large amounts of oxygen molecules are contained in the exhaust gas from the engine, thus making it difficult to clean the nitrogen oxides.
The kind of reaction caused in the exhaust gas catalyzer is shown by the following experiments: composition of the exhaust gas ; CO 2% hydrocarbons 0.04% nitrogen oxides 0.1% CO.sub.2 10% H.sub.2 1% O.sub.2 0-2% H.sub.2 O 10% N.sub.2 remainder
The cleaning efficiency of the CO, hydrocarbons and nitrogen oxides is measured by passing the exhaust gas through a precious metal catalyzer at 400.degree.C and with the space velocity of 30,000H.sup.-.sup.1. The cleaning efficiency is shown in FIG. 1, wherein the cleaned ratio of the constituents in the exhaust gas caused by the oxidation or reduction thereof is shown as the oridinate, whereas the chemical valence ratio of the oxygen is shown as the abscissa. The chemical valence ratio of the oxygen is determined in a basis of an oxygen amount required for burning fully the CO 1 mol. in the exhaust gas. In practice, the chemical valence ratio of the oxygen is indicated as 1.0, when 0.5 mol. of oxygen gas is required for burning fully the exhaust gas containing 1 mol. of CO gas. The larger the chemical valence ratio becomes, the more the oxygen amount is increased. Accordingly, when the oxygen content in the exhaust gas composition is 0 percent, the chemical valence ratio of the oxygen is 0; and when the oxygen content is 2 percent, the chemical valence ratio is 2.
It will be apparent from the graph of the FIG. 1 that efficiency of cleaning the nitrogen oxide is high with the chemical valence ratio lower than 0.6 and reduced sharply with the chemical valence ratio more than 0.6. When the chemical valence ratio is 1.0, the efficiency of cleaning the carbon monoxide, hydrocarbons and nitrogen oxides becomes about 80 percent.
It is considered from test results that the carbon monoxide in the exhaust gas reacts first with the oxygen, and therefore, when only small amounts of oxygen exist in the exhaust gas, the oxygen is fully consumed for oxidizing a part of the carbon monoxide. The nitrogen oxide reacts with the residual carbon monoxide to produce nitrogen gas and carbon dioxide gas. When the oxygen exists in relative large quantities in the exhaust gas, the oxygen oxidizes fully the carbon monoxide and the hydrocarbons. Meanwhile, the carbon monoxide is not reacted with the nitrogen oxides because of the existence of the oxygen, and the nitrogen oxides are scarcely cleaned as the decomposition of the nitrogen monoxide itself.
After the above described experimental works and considerations thereof it has become clarified that, if the exhaust gas is passed through the catalyzer cleaner or the thermal reactor after removing the oxygen from the exhaust gas and converting it into the reducing gas, the nitrogen oxides will be removed from the exhaust gas.